1. Field of the Invention
The present invention relates generally to micro-actuators positioning sliders in a hard disk drive and, more particularly, to a micro-actuator capable of three-dimensional positioning of a slider.
2. Background Information
Modern hard disk drives may have one or more disks. Each disk may have two disk surfaces in use. The hard disk drives also include a servo controller that drives a voice coil actuator to position a read-write head near a track on the surface of a rotating disk. The read-write head communicates with the servo controller, providing feedback, which is used in controlling the read-write head's positioning near the track. The read-write head is embedded in a slider, which floats on a thin layer of air, known as an air bearing, a very short distance above the rotating disk surface.
The trend in the hard disk drive industry is to increase the areal density of the rotating disk surfaces. This is usually achieved by decreasing the flying height of the read-write head above the rotating disk surface, thus reducing the air bearing. Currently read-write heads fly at about 10 nano-meters (nm) from the rotating disk surfaces.
There are problems with flying the read-write heads and sliders so near to the rotating disk surfaces. Occasionally, the read-write heads can contact the disk surface, which tends to reduce the reliability of the data stored on the disk, and possibly damage the read-write head. Contact between the read-write heads and the disk surface they access needs to be minimized to insure the reliability of the hard disk drive.
A voice coil actuator typically includes a voice coil, which swings at least one actuator arm in response signals from the servo controller. Each actuator arm includes at least one head gimbal assembly typically containing a read-write head embedded in a slider. Each disk surface in use has an associated slider, coupled to an actuator arm. The head gimbal assembly couples to a load beam coupled to the actuator arm in the voice coil actuator.
Today, the bandwidth of the servo controller feedback loop, or servo bandwidth, is typically in the range of 1.1 K Hz. Extending servo bandwidth increases the sensitivity of the servo controller to drive the voice coil actuator to finer track positioning. Additionally, it decreases the time for the voice coil actuator to change track positions. However, extending servo bandwidth is difficult, and has not significantly improved recently. As areal densities increase, the need to improve track positioning increases.
One answer to this need involves integrating a micro-actuator into each head gimbal assembly. These micro-actuators are devices typically built of piezoelectric ceramic materials, often including lead, zirconium, and tungsten. The piezoelectric effect generates a mechanical action through the application of electric power. The piezoelectric effect of the micro-actuator, acting through a lever between the slider and the actuator arm, moves the read-write head over the tracks of the rotating disk surface.
The micro-actuator is typically controlled by the servo-controller through one or two wires. Electrically stimulating the micro-actuator through the wires triggers mechanical motion due to the piezoelectric effect. The micro-actuator adds fine positioning capabilities to the voice coil actuator, which effectively extends the servo bandwidth. In the single wire approach, the servo-controller provides a DC (direct current) voltage to one of the two leads of the piezoelectric element. The other lead is tied to a shared ground. In the two wire approach, the servo-controller drives both leads of the piezoelectric element of the micro-actuator.
A problem arises when integrating micro-actuators into hard disk drives with multiple disk surfaces. Each of the micro-actuators requires its leads to be controlled by the servo-controller. These leads are coupled to wires, which must traverse the main flex circuit to get to the flexure. The flexure provides electrical coupling to the leads of the micro-actuator.
The main flex circuit constrains many components of the voice coil actuator. If the shape or area of the main flex circuit is enlarged, changes are required to many of the components of the actuator arm assembly and possibly the entire voice coil actuator. Changing many or most of the components of an actuator arm assembly, leads to increases in development expenses, retesting and recalibrating the production processes for reliability, and inherently increases the cost of production.
The existing shape and surface area of the main flex circuit has been extensively optimized for pre-existing requirements. There is no room in the main flex circuit to run separate control wires to each micro-actuator for multiple disk surfaces. This has limited the use of micro-actuators to hard disk drives with only one active disk surface.
What is needed is a way to minimize the time that the read-write heads fly close to the rotating disk surfaces they access. What is further needed, is a way to integrate micro-actuators into a hard disk drive with multiple disk surfaces, using the existing surface area and shape of the main flex circuit.